1. Field of the Invention
This invention relates to the sensitizing and desensitizing of targets used in electronic article surveillance systems and more particularly it is directed to novel arrangements for controlling the magnetization of high magnetic coercivity desensitizing elements which are spaced apart along the length of such targets.
2. Description of the Related Art
Electronic article surveillance systems of the type with which the present invention may be used are shown and described in U.S. Pat. No. 4,623,877. As shown in that patent, articles of merchandise to be protected from unauthorized taking from a protected area, such as a store or a library, have targets mounted on them and when the merchandise is taken out through an exit from the protected area, the target encounters an alternating magnetic interrogation field which is generated at the exit. The target is an elongated strip of a low coercivity magnetic material which is driven alternately into and out of magnetic saturation by the magnetic interrogation field. This causes the target to disturb the field and to produce magnetic fields which are harmonically related to the original interrogation field. These harmonics are detected and used to generate an alarm.
When a protected article of merchandise is purchased or otherwise authorized to be taken from the protected area, its target must be desensitized so that the merchandise can be taken from the protected area without generating an alarm. One way that has been found very effective is to provide the target with a plurality of spaced apart desensitizer elements or slugs of a high coercivity magnetic material. When these desensitizer elements are magnetized, they prevent the target from generating detectable responses to the interrogating magnetic field. When the desensitizer elements are demagnetized, the target is again made sensitive to the interrogating magnetic fields.
A problem arises in the sensitization of targets. The magnetic hysteresis characteristic of the desensitizer elements is such that when they are subjected to a demagnetization field sufficient to overcome their original magnetic condition, they become remagnetized in the opposite direction. It is necessary therefore to subject the target desensitizer elements to alternating magnetic fields of gradually diminishing intensity. One way of doing this is proposed in U.S. Pat. No. 3,665,449 at Column 5, lines 40-64 and No. 3,765,007 at Column 5, lines 30-56. Specifically it is there proposed to provide either an electromagnet which is subjected to alternate energization at diminishing amplitudes or to provide a series of permanent magnets of successively diminishing strength arranged in a line and passing them over a target desensitizer element.